It is known to make formation pressure measurements at different depth intervals in a wellbore. The classical pressure measurement has been made with a borehole logging sonde. However, the problem with this technique is that the sonde is sensitive to borehole fluid effects. This means that the measured pressure can contain components from both the formation and the borehole fluid but the relative contribution of each can be difficult to determine.
U.S. Pat. No. 7,140,434 discloses a conventional sensor system having a smart plug 11 with embedded sensors 12 inside a cemented casing 16, 18; see FIG. 1. The sensor 12 is sealed in the hole in the casing 16, 18 such that there is no fluid communication between the inside and the outside of the casing 16, 18 through the hole. The sensors 12 are in direct contact with the formation 10 and insulated from the borehole fluids. The plug sealing in the casing wall is key since leakage will affect integrity of the casing 16, 18 and lead to misinterpretation of pressure measurement. The sensor 12 must be insulated from pressure variations inside the casing 16, 18. However, this system still has problems concerning the inference of formation properties.
The production of a well must be monitored and controlled to maximize the production over time since production parameters afford data that define the possible yield of the reservoir. Production levels depend on reservoir formation characteristics such as pressure, temperature, permeability, porosity and the like. In particular, a concern in reservoirs is the inference of formation properties from time varying measurements, for example, monitoring time varying pressure at a number of sensors over a period of time. In this case, sparse measurements of pressure and flow rates in limited number of wells result in an incomplete and uncertain set of measurements. This is attributable to noise in the measurements (particularly reservoir pressure, well production profiles, and water cut in comingled systems) and area heterogeneity. This results in an incorrect inference of formation properties.
The use of interference tests (using single or multiple pressure pulses) for determining formation characteristics such as permeability is now well established. However, almost all applications of interference testing suffer because of the variation of formation properties, particularly permeability distribution, in the vertical direction. Furthermore, the formation pressure signature is usually lost in a multilayer environment when it comes into a comingled wellbore.
While the use of 1D transient well testing (conventional drawdown, buildup, and interference tests) and 3D measurements, made as a function of time as reservoir is depleted, has improved the industry's understanding of well productivity, there still remains a need for an improved inference of formation properties.
There has been a long desire for reservoir engineers to have permanent sensors behind the casing embedded in earth formations. Existing methods of placing permanent sensors behind casing are difficult, cumbersome and not readily applicable. In U.S. Pat. No. 5,467,823 the sensors are placed outside of the casing along with a perforation charge. The sensors communicate with the surface via cables running outside of the casing. The casing and the cables are run in hole and a cementing operation is performed. After cementing, the perforation charges are initiated to perforate through the cement and into the formation so that the sensors communicate with the earth formations. This is a difficult operation and suffers from:
a) cables running outside of the casing, making perforation for production difficult;
b) cement integrity can be questionable, thus the sensors can all be in communication with each other, not reading individual layer properties;
c) the perforation charges may damage sensors and jeopardize cement integrity; and
d) further cased hole logging can be compromised due to cable and sensor presence behind the casing.
An object of the invention is to provide a technique to isolate the reservoir sensor from the wellbore fluids and to effectively and efficiently measure pressure in formations about wellbores for reservoir characterization in oil and gas fields or the like to allow reliable production forecasts and sound reservoir management.
A further object is to attempt to provide a complete set of spatial dynamic measurements for determination of permeability distribution, pressure, and saturation using 4D transient pressure well testing, coupled with the traditional reservoir monitoring about wellbores, particularly open wells.
A still further object of the invention is to define a methodology for pressure sensor placement.
This invention is based on the recognition that it is important to ensure good pressure communication between the sensor and the formation while at the same time isolating the sensor from pressure effects due to the borehole.